The invention disclosed herein is generally related to methods and apparatus for the spectroscopic analysis of shock-compressed materials. More specifically, this invention is related to the analysis of shock-compressed transparent liquids and solids by the use of a technique known as backward-stimulated Raman scattering.
A continuing long-term mission of the Los Alamos National Laboratory is the study of the chemical and physical characteristics of materials at high temperatures and pressures, particularly temperatures and pressures such as those which exist in high explosive detonations. Such characteristics are useful for determining equations of state for the materials and for predicting the behavior of the materials, particularly high explosives, in various configurations and environments. Under such conditions intramolecular and intermolecular forces change considerably and nonequilibrium conditions may be expected. Chemical behavior may be dramatically different from that expected from either extrapolations from ambient conditions or thermodynamic equilibrium calculations. Accordingly, it has been sought to develop fast nonlinear optical techniques to study high-pressure processes which are governed by transient and possibly nonequilibrium phenomena.
For many purposes, it is sufficient and useful to determine the physical and chemical characteristics of materials which are shock-compressed to high pressures by mechanical means rather than by the use of explosives, thus enabling simpler and safer experiments to be conducted under controlled conditions which to some extent simulate the conditions in a high explosive detonation. The present invention is directed to a novel spectroscopic technique which employs such a mechanical shock means, and which is particularly useful for determining vibrational frequencies of shock-compressed materials.
Several optical diagnostic techniques have been previously used to study shock-compressed materials. For example, both emission and absorption spectroscopy have been used for this purpose. However, these techniques suffer from the disadvantage of low signal strengths against high backgrounds. Also, many molecular vibrational transitions occur in the infrared region of the spectrum, where detection systems are not fast enough for the very short time periods available during shock-compression experiments. Optical fluorescence and phosphorescence techniques have also been proposed for the purpose of studying shock-compressed systems, but as yet have only been applied to statically compressed systems.
Several techniques which have been either proposed or actually applied to shock-compressed systems are based on the phenomenon of Raman scattering. Raman scattering is the inelastic scattering of light from molecules. In this regard, light impinging on a molecule is ordinarily scattered elastically, without undergoing any change in frequency, by a scattering process known as Rayleigh scattering. However, a small fraction of the light may undergo inelastic, or Raman scattering at a different frequency. More specifically, in Raman scattering a portion of the energy of the incident photon is typically absorbed by the molecule, resulting in the scattered photon having a lower energy and longer wavelength than that of the impinging photon. In some cases the incident photon absorbs energy from the molecule, resulting in the scattered photon having a higher energy and shorter wavelength than the incident photon.
In both the Rayleigh and the Raman scattering processes, the molecule is momentarily excited by the incident photon to an excited, or virtual, energy level. In Rayleigh scattering the molecule decays back to the initial energy level, whereas in Raman scattering the molecule decays to an excited vibrational level which is typically the v=1 vibrational state. The difference in energy between the incident photon and the emitted Raman photon is equal to the energy difference between the ground vibrational state and the v=1 vibrational state.
The scattering cross-section and hence the detection sensitivity for Raman scattering are considerably smaller than for dipole emission/absorption processes. The small scattering cross-section is particularly significant when the scattering medium has a high background emission level, such as might be the case in a hot shock-compressed material. This difficulty can be overcome to some extent by using a short-wavelength exciting frequency. However, care must be taken to avoid interfering fluorescence from photochemically produced species.
Raman scattering is ordinarily isotropic, i.e., the scattered radiation is emitted uniformly over 4.pi. steradians. However, it has been observed that, when a laser beam is focused in a sample of material, and when the incident laser intensity exceeds a certain threshold level, coherent Raman scattering may occur along the axis of the incident beam. The intensities of these forward and backward directed beams is of considerably greater intensity than that of ordinary Raman scattering as a consequence of laser-like amplification in either direction along the path of the laser beam. Typical threshold intensities of the incident laser beam for stimulated Raman scattering are 10-100 GW/cm.sup.2. As a consequence of the large scattering intensities and the beam-like nature of the scattered signal, there is the possibility of increased detection sensitivity and shorter temporal resolution limits.
A technique that is related to yet different from the present invention was developed by the applicants of the present invention and is disclosed and claimed in the applicants' U.S. patent application Ser. No. 562,150, filed Dec. 16, 1983.
The phenomenon of backward-stimulated Raman scattering is put to use in the method of the invention, which is described below.